The present invention relates generally to ink jet printers and more specifically to a piezoelectrically driven print head for applying ink droplets on demand to a writing surface upon which it is desired to print information. Exemplary of such print heads are those described in U.S. Pat. No. 3,747,120, entitled "Arrangement of Writing Mechanisms for Writing on Paper With a Colored Liquid", and in U.S. Pat. No. 4,106,032, entitled "Apparatus for Applying Liquid Droplets to a Surface by Using a High Speed Laminar Air Flow to Accelerate the Same". Prior art embodiments of ink jet printers have left the ink supply within the print head exposed to the atmosphere or the high speed laminar air flow at all times. While it is necessary during the specific period of time that printing is in progress that the discharge channel be open to allow ejection of the ink, continuous exposure of the ink at other times to the atmosphere or to the high speed laminar air flow is disadvantageous. One undesirable effect is that prolonged exposure tends to dry out the ink supply within the print head. The addition of humectants to the ink supply has been only partially successful as a solution to this problem.
Another undesirable effect of such exposure is dissolution of air into the ink supply. At the high rates at which piezoelectric print heads are driven, air dissolved in the ink supply tends to come out of solution in the form of bubbles, the presence of which in the ink supply seriously degrades performance of the print head. Chemical "getters" have in the past been added to the ink to prevent dissolved oxygen from coming out of solution. However, chemical "getters" have only a limited capacity to bind oxygen and, further, do not prevent bubble formation by gasses other than oxygen.
A further undesirable effect of continuous exposure of the ink supply within the print head to the atmosphere is that air bubbles will sometimes be ingested into the print head through the discharge channel and ink will, at other times, leak out through the discharge channel, depending upon the pressure differential between the ink supply and the atmosphere. In addition, the ink supply may become contaminated by the undesirable introduction of foreign particles through the discharge channel.